1. Field of the Invention
This invention relates to an FRP pipe having a threaded end section at one end or at both ends. More particularly, it relates to a threaded FRP pipe which has male threads or female threads or both and which is suitable for use as piping, tubing, and casing employed in high-pressure environments such as casing or tubing for the production of oil or gas, pipeline for transporting crude oil or natural gas, and piping in various plants.
The term "FRP" as used herein means fiber-reinforced plastics including not only plastics reinforced with glass fibers but also those reinforced with other inorganic or organic reinforcing fibers.
2. Prior Art
FRP pipes are lighter than steel pipes but have high strength and excellent corrosion resistance. In view of these properties, FRP pipes find a wide variety of applications, and in particular are widely used in corrosive environments. For example, they are used as piping in chemical plants and piping for feeding hot spring water or sea water, as well as casing or tubing for the production of oil or gas and pipeline for transporting crude oil or natural gas.
The most important and difficult facet of piping from a technical viewpoint is joints. This holds true for piping using FRP pipes.
Joints which are presently used to join FRP pipes include adhesive-type insertion joints in which FRP pipes inserted therein are bonded with an adhesive, flange joints, threaded joints, and various mechanical joints. Of these, adhesive-type insertion joints are most widely used to join FRP pipes, but they are designed for use in piping to which a relatively low pressure, e.g., 50 kgf/cm.sup.2 or lower, is applied.
For high-pressure piping to which a high pressure, e.g., on the order of 1,000 psi (70 kgf/cm.sup.2) or higher is applied, threaded joints, particularly those prescribed in the specifications of the American Petroleum Institute (API), are widely employed.
FRP pipes for use in high-pressure piping such as crude oil pipelines and oil well tubing are usually produced by the filament winding (hereinafter abbreviated as FW) method. The FW method involves impregnating a bundle of continuous filaments (roving) with a resin (generally a thermosetting resin) and winding the impregnated filaments onto a mandrel under tension. After the resin is cured, the mandrel is extracted leaving a formed FRP pipe.
In the production of an FRP pipe by the FW method, a threaded end section serving as a threaded joint with female (internal) threads can be formed by preparing a mandrel having a threaded end section with male (external) threads which mate with the female threads to be formed in the pipe and winding filaments onto the mandrel along its entire length including the threaded end section according to the usual FW method.
In this case, the filaments are not severed, so it is estimated that the desired high strength can be maintained in the female-threaded joint section. However, it is known that the strength properties of the joint section are largely influenced by the winding angle of the filaments. FIG. 1 shows the relationship between tensile strength of an FRP pipe produced by the FW method and winding angle of filaments with respect to the longitudinal axis of the pipe for various types of filaments. As can be seen from FIG. 1, the tensile strength of an FRP pipe rapidly decreases when the winding angle of filaments exceeds .+-.15.degree.. Therefore, in order to improve the tensile strength of an FRP pipe, it is desirable that the winding angle be in the range of 0.+-.15.degree..
However, when the FW method conventionally employed in the production of high-pressure FRP pipe is merely applied to the formation of a female-threaded joint section of an FRP pipe, the filaments are wound along the thread grooves of the male threads of the mandrel at a winding angle close to 90.degree. with respect to the longitudinal axis of the pipe. Therefore, there was a problem in the prior-art FRP pipe with a female-threaded joint section that if a high internal pressure is applied to the joint section, the female threads with approximately 90.degree. wound filaments are susceptible to cracking due to the component of stress acting in the axial direction.
In order to cope with the problem, a double winding technique is disclosed in Japanese Patent Application Kokai Nos. 56-44625(1981) and 58-45925(1983) in which filaments are wound onto the corresponding-male thread portion of a mandrel initially at an angle between 0.degree. and 30.degree. to the longitudinal axis and subsequently at an angle close to 90.degree. to the axis such that the initially-wound filaments are tightened by the subsequently-wound filaments and forced to descend into the thread grooves of the mandrel.
However, when this double-winding technique is used to form female threads in an FRP pipe having a small thread pitch which is encountered in a threaded pipe joint for oil-well tubing such as a round threaded pipe joint prescribed by API Specification 5B which has 8 or 10 rounds of thread per inch, the initially-wound filaments which are tightened by the subsequently-wound filaments cannot be deflected sufficiently to come into contact with the thread grooves of the mandrel due to the rigidity of the filaments. As a results, voids are frequently formed in the thread grooves, leading to an imperfect shape or contour of the resulting threaded joint section of the pipe.
For this reason, the double winding method disclosed in the above-identified Japanese applications is not applicable to the production of a female-threaded joint of high precision, such as for oil-well tubing which has strict specifications with respect to shape and precision in view of the necessity of maintaining a high degree of seal tightness.
On the other hand, male threads are ordinarily formed in an end section of an FRP pipe by machining the periphery of the pipe in that section after the FRP pipe has been formed by the FW method. Since part of the filaments in the threaded portion are severed by machining, it is difficult to form a male-threaded end section having satisfactorily high strength.
A pipe joint is subject to a tensile force in the axial direction in addition to the internal pressure of the fluid flowing through the pipe. Rupture of a threaded pipe joint caused by an axial tensile force occurs in the form of a shear failure in a thread groove. Therefore, in order to provide a threaded pipe joint having high strength against rupture, it is necessary to strengthen the threaded section against a shearing force in the axial direction. For this purpose it is effective in an FRP pipe joint produced by the FW method to orient the filaments in the radial direction (in the direction of wall thickness), thereby increasing the interlaminar shear strength.
Japanese Patent Application Kokai No. 60-11345(1985) discloses a high-strength male thread prepared from a number of resin-impregnated woven tapes having a width smaller than the diameter of the threaded body. The tapes are gathered and formed into a round rod by means of pultrusion, i.e., drawing them together through a circular die and the resulting rod is then machined to produce male threads on the periphery of the rod. This technique provides male threads having improved axial strength due to the fact that some of the fibers in the weft of the tape are oriented in the radial direction. However, it is directed at male threads on an FRP solid rod formed by pultrusion and it cannot be applied to those on an FRP pipe which is formed by filament winding.
Japanese Patent Application Kokai No. 63-242523(1988) discloses an FRP pipe with a male-threaded end section in which the male threads are formed by wrapping a layer of a resin-impregnated, three-dimensionally woven fabric of reinforcing fibers over the outer periphery of the pipe in an end section thereof and after curing the resin, machining the layer so as to form male threads.
The three-dimensionally woven fabric has vertically-extending fibers across the thickness of the fabric, so the vertically-extending fibers remaining after machining are oriented in the radial direction and contribute to an increase in the interlaminar shear strength (axial strength) of the resulting male threads. However, three-dimensionally woven fabric is expensive and adds to the production costs of an FRP pipe joint incorporating the fabric, which prevents wide use of such joints.